1. Technical Field
The present invention relates to a composition containing uniformly dispersed, finely sized, liquid-state-in-situ-formed ceramic particles in metal and metal alloys, and to products containing the uniformly dispersed, finely sized ceramic particles formed in metal and metal alloys by the liquid-state in-situ process of the present invention. In one aspect, the present invention relates to a composition containing uniformly dispersed, finely sized, liquid-state-in-situ-formed titanium carbide particles in aluminum and aluminum alloys, and to products containing the uniformly dispersed, finely sized titanium carbide particles formed in aluminum and aluminum alloys by the liquid-state in-situ process of the present invention.
2. Background
The aluminum and aerospace industries have long sought a method to control recrystallization of aluminum alloys during deformation operations to permit the design of aluminum airframes with improved structural properties.
The metals industry today conventionally uses dispersoids, i.e., fine particles dispersed in the metal alloy, to control recrystallization and to increase dispersion strengthening at elevated temperatures. Such dispersoids of fine particles dispersed in the metal alloy usually are formed by solid state precipitation.
Recent developments in this area suggest that to improve formability and high temperature strength of aluminum alloys, it is necessary to increase the number densities and to reduce the size of the fine particle size dispersoids.
Conventional processes have the ability to form only a limited level of particle number density, because the number density of the dispersoid is determined by the initial dispersoid forming elements content as limited by its equilibrium solubility in liquid metal during solidification. For example, at the typical solidification rate in the range of about 0.05.degree. C./sec to 20.degree. C./sec, the maximum solubility of zirconium in aluminum is about 0.12 wt. percent, which is considered to be entirely too low for processing at higher temperatures and to form preferred structural properties. Accordingly, a process having the ability to form higher level number densities of stable particle is desired.
In the aluminum industry, dispersoid-forming elements such as Mn, Zr, Cr, V, Ti, Sc, and Hf are added to aluminum to increase the resistance to the recrystallization and increase the recrystallization temperature and to control the grain structure in cast and wrought products. Conventionally, different methods have been employed to add these dispersoid-forming types of alloying elements to molten aluminum metal. Historically, master alloys containing the desired elements have been added directly to the melt in the forms of a solid lump or bar.
The alloying elements in the master alloys normally are present in the form of coarse intermetallics, and these intermetallics require superheat and a long period of holding time to be dissolved in the melt. The heavy intermetallics also tend to settle to the bottom of the holding furnace by force of gravity. For this reason, the master alloys generally are added at a process location up-stream from the molten metal filters so that any coarse intermetallics which do not dissolve in the furnace can be removed prior to casting. If these coarse intermetallics are not filtered out of the molten metal, they adversely affect the mechanical properties of the solidified material. Removal of intentional alloying additions is inefficient and expensive. Perhaps more importantly, however, coarse intermetallic particles do not provide the preferred metallurgical benefits provided by the finer dispersoid particles.
Silicon carbide and alumina are the most commonly used reinforcement particulates. Certain emerging technologies are capable of producing fine particulates of different types with somewhat improved interfacial characteristics. Among the several ways of producing these materials, the technologies where the particles are introduced or formed in the molten aluminum prior to its solidification are attractive, primarily because of the potential for commercially economic processes on a large scale.
A variety of processing routes classified generally as in-situ ceramic phase formation processes in metal have been developed recently. According to the state of the reactants in the process, such a ceramic phase formation process in metal generally is classified into one of several categories:
(1) liquid metal--gas reaction, PA0 (2) liquid metal--liquid metal reaction, or PA0 (3) liquid--solid reaction.
In the case of carbon particles or carbon blocks in the context of liquid metal--liquid metal reactions or liquid--solid reactions, it is known that such carbon particles or carbon black are difficult to introduce directly into a melt in metal because of non-wetting of the carbon by the molten metal or alloy.